Gas turbine engines incorporate combined discs and blades, where the blades are integrally mounted on the disc. These components are known in the art as “blisks”. If the blisks suffer damage during normal service then repair or salvage operations may be required to refurbish them. This typically involves metal deposition or other suitable welding process. The region to be repaired is generally the leading edge of the aerofoil blade of the sections of the blisk blade. A local post repair or post salvage heat treatment process is required to relieve stress of the repair. Also it is often necessary to apply an etching medium to the blisk in order to remove the inherent surface oxide layer.
Etching mediums for gas turbine components are typically acid etching mediums. These mediums can also remove the metal material beneath the surface oxide area. Particularly in the field of gas turbine engines the removal of metal from high value parts is considered disadvantageous given the high tolerance requirements for parts.
A process known as the FFC process uses a molten metallic salt (M2Y) to remove oxides from the surface of articles, such as articles made of titanium, using electrolysis such that reaction of the oxide layer occurs rather than M2 deposition. The known process, described in WO99/64638, places the whole article in the molten metallic salt.
Processing tanks are, on the whole, bulky and require large quantities of chemicals, which have a negative environmental impact. Also, it is often the case that the articles being treated have parts that are more sensitive than others and by placing the whole article within the treatment tank damage may occur to the more temperature sensitive areas.